The present invention relates to a method and apparatus for precisely positioning a plurality of optical fibers, and more particularly, to a method and apparatus for inserting a plurality of optical fibes into the bore of a capillary tube.
A plurality of optical fibers may need to be precisely positioned while manufacturing a device that includes the fibers. To form an overclad fiber optic coupler, for example, the stripped portions of a plurality of fibers are inserted into a capillary tube to form a coupler preform. The midregion of the coupler preform is then heated and collapsed onto the fibers and stretched until the desired coupling characteristics are obtained. The time required for manually inserting fibers into the tube can be a significant portion of the time required to make the coupler. Also, the reproducibility of the coupler manufacturing operation is dependent to some extent upon the positioning of the fibers in the tube.
It is especially difficult to insert the fibers into the capillary tube when making 1.times.N couplers when N.ltoreq.6. See U.S. Pat. No. 5,268,979 for fiber orientations for 1.times.6 and 1.times.8 couplers. To make a 1.times.6 coupler, for example, six fibers are disposed around a central fiber in the coupling region.
It is even more difficult to make multifunction overclad couplers of the type disclosed in U.S. patent application Ser. No. 08/565,542 (D. L. Weidman P00007) filed now U.S. Pat. No. 5,664,037 on even date herewith. Such couplers have two or more tapered coupling regions, as further discussed below in conjunction with FIGS. 9, 10 and 11. In certain 1.times.N multifunction coupler embodiments, wherein N.ltoreq.6, the end of an active fiber having a core and cladding may butt against a dummy fiber that does not propagate light, the junction between the fibers being somewhere between two of the coupling regions. By active fiber is meant an optical fiber having a core and cladding; such a fiber is capable of efficiently propagating light. Active fibers are contrasted with "dummy" fibers that do not contain cores. This relationship is used to prevent two adjacent fibers that have coupled in a first coupling region from coupling in another coupling region. If such coupling did occur, the resultant interference effects would be similar to those that occur in Mach-Zehnder devices, and the output power would vary as a function of wavelength. During the process of making such a coupler, fibers are inserted into the capillary tube bore so that some of the fibers extend entirely through the tube, and some of them are composite fibers consisting of an active fiber and a dummy fiber which abut one another within the tube. If the composite fibers are not made by fusing together the active and dummy fibers, then the device is made by inserting one part of the composite fiber into one end of the tube and another part of the composite fiber into the other end of the tube. Dummy and active fibers need to be precisely positioned at both ends of the tube if they are to become aligned end-to-end.
In accordance with this invention the handling of optical fibers is facilitated by providing a method and apparatus for precisely positioning a plurality of optical fibers relative to a coupler forming apparatus and/or other elements such as capillary tubes that are used in the manufacture of fiber optic devices.
When a coupler preform is formed off-line and thereafter inserted into the coupler draw apparatus, the fiber pigtails extending from the capillary tube are tacked to the ends of the tube so that the bare regions of the fibers remain in the proper position within the tube bore during the coupler preform transfer step. The epoxy that is used to tack the fiber pigtails can have a detrimental effect on the resultant coupler. Such detrimental effect can be avoided by forming the coupler preform while the capillary tube is in the coupler forming apparatus. This procedure would save time since it eliminates the step of transferring the coupler preform from the off-line fiber insertion apparatus to the coupler draw apparatus. A further reduction of coupler production time can be achieved by automatically feeding fiber to the draw apparatus. When forming couplers on-line in the coupler draw apparatus, various fiber modification steps such as coating stripping and fiber end termination are performed at or near the draw apparatus. After such fiber modification steps have been performed on one or more fibers but before those fibers have been inserted into the capillary tube, they can clutter the coupler draw apparatus. Moreover, even after the fibers have been inserted into the capillary tube, the pigtails extending from the capillary tube can clutter and interfere with the coupler draw process. A further feature of the invention is to dispense optical fiber to a coupler draw apparatus in such a manner as to avoid the aforementioned problems.